Introduction

The prediction of gas well deliverability under various operating conditions including variations in spacing and reservoir depletion has always caused problems. Many years ago it was observed that in most cases a plot of the log of the producing rate versus the log of the static reservoir pressure squared minus the square of the flowing well pressure. log (Pe2-pw 2), resulted in a straight line. It has since been shown that theoretically such a straight line is obtained only when pseudo steady state flow is obtained at each rate which means that each rate must be maintained until the effects of that rate have been felt throughout the drainage area of the particular well. If this condition is not met, the apparent straight line that is obtained does not represent the slope that-would be obtained from stabilized (pseudo steady state) data. Furthermore, there is generally a great difference between the position of the deliverability curve for the uncorrected flow after flood type data and the deliverability curve that would be obtained for stabilized flow. These points are illustrated in Figure 1. Note that "pseudo steady state" and "stabilized conditions" are used interchangeably in this paper.

The problem is that in most gas reservoirs the stabilization time is excessive. In some tight widely spaced, commercially produced wells it can be shown that stabilization is not obtained after one year's time. Thus, it becomes necessary to test the well under infinite acting conditions and interpret the data in terms of the performance that would be obtained after stabilized conditions are reached The isochronal, technique was proposed for performing such infinite acting tests and performing such infinite acting tests and interpreting the data in terms of stabilized data. Ideally this technique involves producing at a particular rate for some relatively short particular rate for some relatively short period of time, shutting the well in until the period of time, shutting the well in until the reservoir pressure reaches static conditions, initiating production again at a different rate for the same length of time, shutting the well in until the static pressure distribution is again obtained in the reservoir, producing at a third rate for the same length of time, shutting the well in … etc. Theoretically, this technique has much to offer and a correct application of the data will provide the desired predictions. However, in practice the engineer predictions. However, in practice the engineer is always faced with the problem of how long to shut the well in between flow periods.

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